Communication services have experienced a fast growing demand, for example, generalization of information communication services, introduction of various multimedia services, and provision of high-quality services. To meet such a demand, various wireless communication techniques have been studied in various fields.
According to service types, multimedia data requires a variety of data rates and a variety of transmission qualities. Therefore, there is a need for a new link adaptation scheme having a different concept from the conventional voice-oriented services. An example of the link adaptation scheme for effective data transmission is an adaptive modulation and coding (AMC) scheme in which a data rate is adaptively changed to fit a channel environment. The data rate is determined by a modulation and coding scheme (MCS) level in association with a predetermined modulation and channel coding combination. The MCS level is determined according to a signal to interference plus noise ratio (SINR), a carrier to interference plus noise ratio (CINR), channel quality information (CQI), etc. To support the AMC scheme, a base station (BS) has to know information on the SINR of a user equipment (UE), and the UE informs the BS of receive (Rx) channel quality by using the CQI or the like.
A multiple input multiple output (MIMO) technique is one of techniques capable of improving communication capacity and communication performance without additional frequency allocation or power increase. That is, spectral efficiency can be improved by increasing both the number of transmit (Tx) antennas and the number of Rx antennas since channel transmission capacity is theoretically increased in proportion to the number of antennas.
The MIMO technique can be classified into a spatial diversity scheme and a spatial multiplexing scheme. The spatial diversity scheme uses various channel paths to increase transmission reliability. The spatial multiplexing scheme improves the data rate by simultaneously transmitting a plurality of data streams. In recently conducted researches, the two schemes are properly combined to obtain advantages of each scheme.
A precoding scheme (or a beamforming scheme) may be used to increase the data rate. When a transmitter intends to transmit data by using the precoding scheme, each data column is multiplied by a Tx weighting factor by using information on a downlink channel from the transmitter to a receiver. Therefore, the transmitter has to know a condition of a downlink channel from each Tx antenna of the transmitter to each Rx antenna of the receiver. For this, the receiver estimates the downlink channel condition, and thereafter feeds back information on the estimated downlink channel condition to the transmitter. However, in order to feed back the downlink channel condition information to the transmitter, a large amount of feedback data has to be transmitted by using an uplink feedback channel.
Therefore, a codebook scheme is introduced to reduce the amount of feedback information. In the codebook scheme, the receiver determines a precoder having a maximum data rate by using a precoder codebook consisting of a limited number of precoders known by both the transmitter and the receiver, and feeds back an index of the precoder to the transmitter. The transmitter transmits data by using a precoder corresponding to the fed-back index of the precoder included in the precoder codebook.
The transmitter having multiple antennas can simultaneously transmit a plurality of Tx signals by broadcast, multicast, and/or unicast transmission. The broadcast transmission denotes that the signals are transmitted to all users within a specific region (e.g., a cell and/or a sector). The multicast transmission denotes that the signals are transmitted to a specific group consisting of at least one user. The unicast transmission denotes that the signals are transmitted to a specific user.
In a point-to-point system such as unicast transmission, precoding is not much problematic. However, since services are collectively provided to a plurality of users in a point-to-multipoint system such as multicast or broadcast transmission using multiple antennas, feedback overhead may occur when each user feeds back separate precoding information and MCS level information to the BS. Moreover, since the MCS level and the precoder are different from one user to another, it is difficult to perform scheduling for suitable coding and beamforming.
Accordingly, when multi-user data is transmitted using multiple antennas, there is a need for a method of effectively transmitting feedback information (e.g., the MCS level, the CQI, or the precoder) to a BS.